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How to: Implement Validation Logic on Custom Objects

This example shows how to implement validation logic on a custom object and then bind to it.

You can provide validation logic on the business layer if your source object implements IDataErrorInfo, as in the following example:

	Public Class Person
        Implements IDataErrorInfo

        Private _age As Integer
        Public Property Age() As Integer
            Get
                Return _age
            End Get
            Set(ByVal value As Integer)
                _age = value
            End Set
        End Property

        Public ReadOnly Property [Error]() As String Implements IDataErrorInfo.Error
            Get
                Return Nothing
            End Get
        End Property

        Default Public ReadOnly Property Item(ByVal columnName As String) As String Implements IDataErrorInfo.Item
            Get
                Dim result As String = Nothing

                If columnName = "Age" Then
                    If Me._age < 0 OrElse Me._age > 150 Then
                        result = "Age must not be less than 0 or greater than 150."
                    End If
                End If
                Return result
            End Get
        End Property
    End Class

In the following example, the text property of the text box binds to the Age property of the Person object, which has been made available for binding through a resource declaration that is given the x:Key data. The DataErrorValidationRule checks for the validation errors raised by the IDataErrorInfo implementation.

<TextBox Style="{StaticResource textBoxInError}">
    <TextBox.Text>
        <!--By setting ValidatesOnExceptions to True, it checks for exceptions
        that are thrown during the update of the source property.
        An alternative syntax is to add <ExceptionValidationRule/> within
        the <Binding.ValidationRules> section.-->
        <Binding Path="Age" Source="{StaticResource data}"
                 ValidatesOnExceptions="True"
                 UpdateSourceTrigger="PropertyChanged">
            <Binding.ValidationRules>
                <!--DataErrorValidationRule checks for validation 
                    errors raised by the IDataErrorInfo object.-->
                <!--Alternatively, you can set ValidationOnDataErrors="True" on the Binding.-->
                <DataErrorValidationRule/>
            </Binding.ValidationRules>
        </Binding>
    </TextBox.Text>
</TextBox>

Alternatively, instead of using the DataErrorValidationRule, you can set the ValidatesOnDataErrors property to true.

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